The present invention is concerned with an audio codec supporting noise synthesis during inactive phases.
The possibility of reducing the transmission bandwidth by taking advantage of inactive periods of speech or other noise sources are known in the art. Such schemes generally use some form of detection to distinguish between inactive (or silence) and active (non-silence) phases. During inactive phases, a lower bitrate is achieved by stopping the transmission of the ordinary data stream precisely encoding the recorded signal, and only sending silence insertion description (SID) updates instead. SID updates may be transmitted in a regular interval or when changes in the background noise characteristics are detected. The SID frames may then be used at the decoding side to generate a background noise with characteristics similar to the background noise during the active phases so that the stopping of the transmission of the ordinary data stream encoding the recorded signal does not lead to an unpleasant transition from the active phase to the inactive phase at the recipient's side.
However, there is still a need for further reducing the transmission rate. An increasing number of bitrate consumers, such as an increasing number of mobile phones, and an increasing number of more or less bitrate intensive applications, such as wireless transmission broadcast, necessitate a steady reduction of the consumed bitrate.
On the other hand, the synthesized noise should closely emulate the real noise so that the synthesis is transparent for the users.